A known particle separating apparatus is disclosed in, for example, Japanese Unexamined Patent Publication No. 5-277402. This apparatus is provided with a stock solution supply passage for supplying a stock solution containing particles, a gas injector for injecting a gas into the stock solution in the supply passage while dispersing it, a particle separator, a concentrated solution discharge passage for discharging the concentrated solution in the particle separator, and a clear solution discharge passage for discharging the clear solution in the particle separator. The particle separator swirls the stock solution flown from the supply passage to create a static pressure difference in radial directions, moves gas particles in the stock solution toward the center by the static pressure difference to generate a gas-liquid mixed swirl at the center, and concentrates the particles at one end of the longitudinal direction of the swirl by this gas-liquid mixed swirl to thereby separate the stock solution into a clear solution and a concentrated solution.
According to a centrifugal force and the static pressure difference created by swirling the stock solution, the particles contained in the stock solution and having larger specific gravities than water are moved toward the outside of the swirl and discharged to the outside via the concentrated solution discharge passage provided at a lateral side, whereas the particles contained in the stock solution and having smaller specific gravities than water are moved toward the center and then moved upward together with air bubbles generated at the center to be discharged via the clear solution discharge passage provided at an upper side.
In the particle separating apparatus thus constructed, the particles having larger specific gravities than water and those having smaller specific gravities than water can be effectively separated. However, colloidal or gelatinous particles having specific gravities similar to that of water cannot be effectively separated. It is difficult to prevent these particles from being mixed into the clear solution and admitted into the clear solution discharge passage. Thus, this apparatus has a problem of being unable to sufficiently separate the particles if a large quantity of particles having specific gravities similar to that of water are contained in the stock solution.
In the particle separating apparatus disclosed in the above publication, an outer tube member forming an outer casing of the apparatus is provided. This outer tube member is comprised of a large-diameter portion located below for installing an inner tube member forming the particle separator and a small-diameter portion located above to be connected with an upper member where the concentrated solution discharge passage is provided. Thus, this apparatus has additional problems of the complicated construction of the outer tube member and difficulty to manufacture the outer tube member.